Optically pumped solid state Nd:YAG slab lasers are an important class of solid state lasers. One type of Nd:YAG slab laser is a zigzag slab geometry, illustrated in FIG. 1. In the zigzag slab geometry the Nd:YAG slab has a rectangular cross-section. The slab includes tilted facets to permit a laser beam to enter and exit the slab. The slab has two large opposing faces. As illustrated in prior art FIG. 1, the slab is optically pumped by illuminating it throughout its entire length through the opposing faces. The angle of the tilted facets results in the beam entering the slab an angle (due to Snell's law) and then experiencing total internal reflection at the opposing faces. By proper selection of the angle of the titled facets and other design factors the beam zigzags down the length of the slab due to total internal reflection. That is, the optical beam experiences a sequence of internal reflections such that it has an alternating path along the length of the slab in which the beam bounces back and forth between the two large opposing faces. Additional retro-reflectors are provided to provide feedback to the slab laser. Background information on the traditional zigzag solid state laser geometry is described in W. Koechner, Solid State Laser Engineering, Springer Verlag Ed., 1976, p. 392-394, B. J. Comaskey et. al., “High average power diode pumped slab laser”, IEEE J. Quantum Electronics, vol. 28, no 4, April 1992, p. 992, and A. D. Farinas et, al., “Design and characterization of a 5.5-W, cw, injection-locked, fiber-coupled, laser-diode-pumped Nd:YAG miniature-slab laser, Optics Letters, vol 19, no. 2, Jan. 15, 1994, p. 114, the contents of which are hereby incorporated by reference.
One aspect of a conventional Nd:YAG slab zigzag laser is that it is optically pumped throughout the entire body of the slab. Thus, the beam experiences optical gain along the entire length that it traverses the slab. Additionally, a conventional Nd:YAG zigzag slab laser benefits from the comparatively small gain spectrum and comparatively small variation in refractive index characteristics of solid state lasers. As a result, a conventional Nd:YAG slab laser can be used to provide a high quality laser output.
In contrast, a zigzag laser geometry has previously proven impractical for implementing an efficient, high power semiconductor laser with desirable optical mode characteristics. Semiconductor lasers have a comparatively broad gain spectrum, higher gain coefficients, and a high sensitivity of refractive index to temperature and pumping level. It is difficult to achieve high power output from large area and volumes of gain material without creating various optical instabilities in the spatial and longitudinal modes. For example, it is well known that a slab-geometry semiconductor laser pumped over a large width has a tendency to lase in multiple lateral modes. That is, special efforts are often required to suppress parasitic lasing in slab-geometry semiconductor lasers and amplifiers because of the large gain in typical semiconductors. For example, lateral lasing can occur in high fill factor laser bars unless deep isolation trenches are etched between the emitter stripes to circumvent it. As a result, because of the large gain in typical semiconductor materials, the design to suppress parasitic lasing is a common issue. There are also practical difficulties in modulating a large area semiconductor laser due to their large capacitance (large RC time constant), the requirement for high modulation currents and the generation of various modal instabilities.
There are several semiconductor zigzag lasers that have been proposed in the prior art. However, each of them has significant shortcomings that make them unusable as a practical alternative to other types of semiconductor lasers.
One approach to implementing a zigzag semiconductor laser is disclosed in U.S. Pat. Pub. 2003/0012246 by Klimek, “Semiconductor ZigZag Laser and Optical Amplifier.” However, the numerous deficiencies in Klimek make the design unusable for achieving an efficient, electrically pumped laser with a stable large-area optical mode. The epitaxial layer design approach disclosed in U.S. Pat. Pub. 2003/0012246 is extremely close in design to conventional solid state slab lasers. The semiconductor structure includes an active region sandwiched between two cladding layers, similar to a conventional edge-emitting semiconductor laser. The active region includes p-doped and n-doped regions. Klimek describes both optical pumping and electrical pumping but does not disclose a practical means to provide efficient electrical pumping. The embodiment with electrical pumping would be understood to require p-doping in one of the cladding layers and n-doping in the other cladding layer in order to form a p-n laser diode between the top and bottom electrical contacts. Thus, in electrically pumped embodiments of Klimek the optical mode traverses a zigzag path through both n and p doped active regions and n and p doped cladding regions, resulting in significant optical loss. Moreover, the zigzag path has a sequence of nodes and antinodes along the active region. As a result gain is not utilized effectively.
Additionally, there are also several other practical problems with the electrically pumped embodiments disclosed in Klimek. Klimek discloses that the optical cladding layers are epitaxially grown. However, there are practical limits to the thickness with which doped cladding layers can be grown with a high optical and material quality and with a reasonably low electrical resistance consistent with an efficient design. In comparable edge-emitting laser structures the cladding regions are typically only a few microns in thickness to reduce electrical resistance and to minimize undesirable optical losses in doped layers. However, if such an efficient laser structure was used in Klimek it would limit the vertical mode to a few microns in dimension, i.e., to a dimension consistent with bouncing the mode within a laser structure having cladding layers of only a few microns in thickness. Thus, an efficient electrically pumped implementation of Klimek would have a small spot size in at least one dimension because of limitations on the cladding thicknesses. Klimek also does not disclose a practical means to couple light into and out of the semiconductor chip. While Klimek discloses one embodiment with angled facets, such angled facets are difficult to fabricate with controlled angles in most semiconductors. In particular, the natural cleavage and etching planes of most semiconductors do not occur at the same angles required for a slab laser. Another embodiment in Klimek utilizes micro-prism elements to couple light to the semiconductor chip. However, such micro-prisms would be difficult to implement on such a small physical scale and would require additional alignment and packaging steps that are undesirable. Additionally, Klimek teaches that the slab has a large lateral size. However, this will tend to result in the generation of multiple lateral modes in the laser, similar to that observed in large area edge-emitting lasers, or as one extreme would result in parasitic lateral lasing. No teaching is provided in Klimek regarding how parasitic lasing would be suppressed. Klimek also does not provide guidelines for practical dimensions of its proposed structures and offers no solutions for dissipating and removing heat from the laser and for addressing stresses in the epitaxial structure. Klimek also has other deficiencies, including non-operative examples in which the stated materials would not have the correct indices of refraction to provide total internal reflection.
Another approach to implementing a semiconductor laser having a zigzag-like path is described in U.S. Pat. No. 5,131,002 by Aram Mooradian entitled “Optically-pumped external cavity laser” (hereinafter “The Mooradian patent”). The Mooradian patent discloses optically pumping a semiconductor material in a series of active regions of a semiconductor wafer using an array of optical pump sources. Each active region includes an active semiconductor laser material, such as GaAs, that is sandwiched between cladding layers of AlGaAs. The epitaxial structure is described as having a thickness of a few microns or tens of microns. The Mooradian patent does not, however, utilize total internal reflection. Instead, the zigzag path is generated by utilizing two additional mirrors. A bottom mirror is formed on the bottom of the substrate. Thus, the optical mode in Mooradian must traverse all of the semiconductor layers and the substrate in order to reflect off the bottom mirror. A second (top) mirror is positioned above the wafer. No description is provided in the Mooradian patent how the active regions would be electrically pumped. Moreover, the Mooradian patent is also silent regarding various other details necessary to make an efficient laser with a stable optical beam. Some of the deficiencies of the Mooradian patent are described in U.S. Pat. Pub. No. 2006/0251141 by Mefferd et al., which describes in paragraphs [0008]-[0009] that the Mooradian patent suffers from deficiencies such as problems in cooling an extended pumped area, potential misalignment due to changes in flatness over the chip, and the need to vary the areas of the active regions by up to a factor of four to achieve an efficient resonator. U.S. Pat. Pub. No. 2006/0251141 in fact proposes using a complex structure having multiple separate chips and external fold mirrors to solve these problems.
U.S. Pat. Pub. No. 2006/0176544 by Wasserbauer discloses an optical amplifier structure in which light travels in a zigzag path along the length of the optical amplifier. The optical amplifier is described as being uniformly pumped and provided with a transverse waveguide, such as a ridge waveguide. The active region in sandwiched between top and bottom distributed Bragg reflector mirrors. The optical beam enters the amplifier and strikes the bottom reflector, passes through the gain region, bounces off the top mirror, and then is directed down to the bottom mirror and so on until the optical mode has traveled through the entire length of the amplifier. However, this amplifier design has various shortcomings. First, the use of optical gain is potentially inefficient, as the entire amplifier is pumped whereas the zigzag path will result in a sequence of nodes and antinodes along the gain region due to the zigzag path. Second, the spot size will be limited to a comparatively small spot size. In the lateral dimension the spot size will be limited by the transverse lateral guide to dimensions of about a few microns or so. Second, in the vertical dimension the spot size will also be limited to a few microns or so, consistent with the thickness of the epitaxial mirrors. In light of these various considerations, the amplifier cannot be used as the basis of an efficient, high-power laser with a large spot size.
Thus, while the prior art suggests a long-felt need for a zigzag-type semiconductor laser with desirable beam characteristics this objective has not been met. The solutions proposed in the prior art are not consistent with achieving an electrically pumped semiconductor laser that is efficient, capable of high power output, which has a large spot size and which can be effectively used in a gain-switched or mode-locked configuration.